Distributed resonant ring fiber filter

ABSTRACT

Disclosed is a fiber optic filter that includes a central core, a ring core concentric with the central core, an inner cladding region of refractive index n i  between the central and ring cores, and a cladding layer of refractive index n, surrounding the ring core. The maximum refractive index n, of the central core and the maximum refractive index n 2  of the ring core are greater than n c  and n i . The propagation constants of one core mode and one ring mode are different at wavelengths except for at least one wavelength λ 0  whereby power transfers between the two cores at λ 0 . At least a portion of the fiber optic filter fiber is wound around a reel to subject it to a continuous curvature, the radius of which determines the amplitude of the attenuation. Such fibers are useful in fiber amplifiers in which the central core contains active dopant ions.

This application claims the benefit of U.S. Provisional Application No.60/117,080 filed in Jan. 25, 1999.

BACKGROUND OF THE INVENTION

The present invention relates to fiber amplifiers having filter meansfor attenuating or removing certain specified wavelengths, and toresonant ring fiber filters for use in such amplifiers.

Doped optical fiber amplifiers consist of a gain fiber the core of whichcontains a dopant such as rare earth ions. The gain fiber receives anoptical signal of wavelength λ_(s) and a pump signal of wavelength λ_(p)which are combined by means such as one or more couplers located at oneor both ends of the gain fiber. The spectral gain of a fiber amplifieris not uniform through the entire emission band. For example, an erbiumdoped gain fiber, the gain band of which coincides with the 1550 nmtelecommunications window of silica based optical fiber, has anirregular gain spectrum that includes a narrow peak around 1536 nm.Fiber amplifier gain spectrum modification has been employed in fiberamplifiers for such purposes as gain flattening and gain narrowing.

It is known that a gain fiber can include a distributed filter forimproving the efficiency of a fiber amplifer and/or tailoring thespectral output thereof. Such a distributed filter/gain fiber has anactive ion-doped core that is located along the fiber axis, and itfurther includes a second, off-axis core that extends parallel to theactive ion-doped core. The two cores have different characteristics suchas core diameters and/or refractive index profiles. The structure cansupport at least two core modes, and the propagation constants of thetwo core modes can be manipulated independently by proper selection ofthe aforementioned characteristics. The cores can therefore be designedsuch that their propagation constants are equal at a certain resonantwavelength, λ₀. At wavelength λ₀ the fundamental mode of the structurechanges from one core to another. Strong power transfer between the twocores can happen only at a narrow band of wavelengths centered about theresonant wavelength. If the second core contains a light absorbingmaterial, it will absorb at least a portion of the light centered aboutwavelength λ₀ to provide a filtering function that modifies the fiberamplifier gain spectrum.

It is difficult to make a fiber having two parallel cores because of itslack of circular symmetry. Also, a filter having two parallel cores ispolarization dependent.

These disadvantages could be avoided by providing the amplifier with aknown coaxial coupler of the type wherein a ring core is concentric withand radially spaced from the central active core to form a devicereferred to herein as a resonant ring fiber (RRF). At least two modesexist in a RRF. Any mode with most of its power in the core is definedas a core mode, and any mode with most of its power in the ring isdefined as a ring mode. The propagation constants of one of the coremodes and one of the ring modes of a RRF can be manipulatedindependently by varying the parameters of the core and ring. The twomodes of the RRF structure behave in the same way as the two modes inthe parallel core fiber coupler/filter described above, but the RRF ismuch easier to make using vapor deposition-based conventional fiberfabrication technology; moreover, it is intrinsically not polarizationdependent due to its circular symmetry.

In the aforementioned parallel core coupler/filter, differing amounts ofpower can be attenuated in the off-axis core, depending upon theconcentration of light absorbing dopant material contained in thatoff-axis core. After the parallel core filter is made, the amount ofattenuation per unit length therein is fixed. If manufacturingtolerances were such that a predetermined length of fiber did notprovide the desired attenuation, it would be desirable to be able totune the attenuation to the desired value.

SUMMARY OF THE INVENTION

An object of the invention is to improve the efficiency of a fiberamplifier and/or tailor the spectral output of a fiber amplifier.Another object is to provide an improved fiber optic filter. Yet anotherobject is to provide a distributed fiber optic filter, the peak filterwavelength and peak attenuation of which can be readily adjusted.Another object is to provide a temperature stable fiber optic filter.

The present invention relates to a distributed filter formed of anoptical fiber having a central core, a ring core having an inner radiusr_(R) concentric with the central core, an inner cladding region ofrefractive index n_(i) between the central and ring cores, and acladding layer of refractive index n_(c) surrounding the ring core. Themaximum refractive indices n₁ and n₂ of the central and ring cores aregreater than n_(c) and n_(i). At least a portion of the optical fiber issubjected to a continuous curvature as by winding it into a coil. Thepropagation constants of one core mode and one ring mode are differentat wavelengths except for wavelength λ₀, whereby a narrow band ofwavelengths including λ₀ is coupled between the central core and thering core and is at least partially radiated, whereby the narrow band ofwavelengths is attenuated.

This technology is especially useful for implementation of distributedloss filters in gain fibers utilized in certain fiber amplifier andlaser designs where an appropriate ring core is used in addition to theconventional active ion-doped central core to obtain spectral gainshaping. The ring structure can be designed to provide the appropriateloss for a certain fiber coil size at those wavelengths where the fiberamplifier exhibits amplified spontaneous emission.

The peak attenuation wavelength λ₀ of a fiber of given outside diametercan be measured, and it may be determined that a fiber having adifferent value of r_(R) (and thus outside diameter) will result in thecorrect value of wavelength λ₀. Thereafter, the draw blank can be drawnto a fiber having an outside diameter different from the given outsidediameter. It may be beneficial to add more cladding material to theoriginal draw blank or to etch some cladding material from the originaldraw blank prior to drawing the modified resonant ring fiber; thesesteps could result in a fiber having a different value of r_(R) and yetretain the given outside diameter. Also, a drawn fiber can be stretchedto decrease its outer diameter.

The cladding portions of the fiber can consist of a base glass such asSiO₂ and the central and ring cores can comprise SiO₂ doped withdifferent amounts of a refractive index increasing dopant such as GeO₂or Al₂O₃ to increase the refractive index. The filter can beathermalized by employing an appropriate co-dopant such as B₂O₃ togetherwith the index raising dopant to balance out the thermal dependence ofthe propagation constants of the central and ring cores.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an optical amplifier using a resonantring gain fiber.

FIG. 2 is an enlarged schematic and fragmentary view of a fusion splicebetween two single-core fibers and a resonant ring gain fiber.

FIG. 3 schematically shows a further embodiment of a coiled resonantring gain fiber.

FIGS. 4 and 5 show two structures for imparting continuous curvature toparts of a resonant ring gain fiber.

FIG. 6 is a graph showing fiber amplifier gain spectra.

FIGS. 7 and 8 are exemplary refractive index profiles of the resonantring fiber of this invention.

FIG. 9 is a graph illustrating the behavior of the normalizedpropagation constants of the LP01 and LP02 modes of a resonant ringfiber.

FIG. 10 is a graph illustrating the wavelength dependence of modal fielddistribution for the LP01 mode.

FIG. 11 is a graph wherein normalized field for the LP01 and LP02 modesis plotted as a function of radius at resonance.

FIG. 12 is a measured refracative index profile of a resonance ringfiber.

FIG. 13 is a graph illustrating the dependence of resonance wavelengthon fiber diameter for a plurality of fibers drawn from the same fiberdraw blank.

FIG. 14 illustrates an experiment for determining the effect ofcurvature on attenuation.

FIG. 15 is a graph illustrating the dependence of filter performance oncoil diameter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fiber amplifiers, in which useful gain is afforded by the stimulatedemission of radiation, are employed for the purpose of amplifying asignal propagating in the transmission line fibers of opticaltransmission systems. The basic structure of a fiber amplifier isschematically illustrated in FIG. 1. A signal of wavelength λ_(s) hasbecome attenuated after propagating through a length of transmissionline fiber 14. Such signal is applied to a wavelength dependentmultiplexer coupler 11 where it is combined on a single outgoing fiber12 with pumping energy of wavelength λ_(p) generated by source 13. Fiber12 is connected by fusion splice 16 to a first end of a gain fiber 10,the central core of which contains active dopant ions. The amplifiedsignal propagates from gain fiber 10 to a single-core transmssion linefiber 15 that is connected to she second end of fiber 10 by fusionsplice 17.

The portions of fibers 10, 12 and 15 adjacent fusion splices 16 and 17are shown in FIG. 2. Fibers 12 and 15 are conventional single-coreoptical fibers. Fiber 12 includes a core 23 surrounded by a claddinglayer 24, and fiber 15 includes a core 25 surrounded by a cladding layer26. Splices 16 and 17 are represented by dashed lines. For the sake ofsimplicity, protective coatings are not shown. Fiber 10 is a resonantring fiber having a central core 19 and a ring core 20, the two coresbeing separated by inner cladding material 21. Fiber 10 also includes alayer of outer cladding material 22. The significance of the resonantring structure will be discussed below.

Whereas the end-to-end abuttment of fiber 10 with fibers 12 and 15 hasbeen illustrated as being fusion splices, mechanical connectors couldalso be employed.

FIG. 1 illustrates that fiber 10 is wound upon a reel 18, splices 16 and17 being illustrated as being located at a distance from the reel.Alternatively, one or both of the splices 16 and 17 could be located onthe reel. One such embodiment is shown in FIG. 3 where elements similarto those of FIG. 1 are represented by primed reference numerals. In FIG.3, splices 16′ and 17′ are located along the surface of reel 18′ wherebythe entire length of resonant ring gain fiber 10′ is subjected to thecurvature of the reel. Short end portions of fibers 12′ and 15′ are alsolocated on reel 18′.

The resonant ring gain fiber could be subjected to curvatures other thancircular. For example, a resonant ring fiber 30 could be wound in anS-shaped pattern about two circular supports 31 and 32 as shown in FIG.4. In the embodiment of FIG. 5, a resonant ring fiber 35 is wound in a“racetrack” pattern about two circular supports 36 and 37.

Although the present invention is useful with various kinds of fiberamplifiers, it will be described in conjunction with an erbium-dopedfiber amplifier because of its utility in communication systems. Asshown by curve 40 of FIG. 6, the gain spectrum of an erbium-doped fiberamplifier has a narrow peak around 1536 nm and a broad band with reducedgain to about 1560 nm. The 1536 nm peak must be reduced to prevent theoccurrence of such disadvantageous operation as wavelength dependentgain or gain (with concomitant noise) at unwanted wavelengths. As willbe described more fully below, the coiled, resonant ring aspect of gainfiber 10 provides fiber 10 with a distributed optical filter functionthat can be designed to attenuate the peak gain of the fiber amplifier.By suppressing the peak gain, the amplifier noise is reduced throughreduction of spontaneous-spontaneous beat noise in addition to achievingthe desired flat gain spectra required in a wavelength divisionmultiplexing system. Owing to the reduced chance of lasing action fromthe peak gain, the amplifier can also be designed to operate at muchhigher population inversion; this can have an additional effect onreduction of noise. This results in the uniform gain spectrumrepresented by curve 42 of FIG. 6.

If the curved, resonant ring structure of fiber 10 were designed toattenuate substantially all of the power at 1536 nm, the resultantfiltering would essentialy eliminate the shorter wavelengths from theerbium spectral gain curve, thereby resulting in a spectral gain of thetype represented by curve 42 of FIG. 6.

As shown in FIG. 2, the resonant ring distributed filter/gain fiber 10includes a concentric ring core 20 in addition to the central core 19.In the idealized exemplary refractive index delta profile of FIG. 7,central core 48 is separated from ring core 50 by inner cladding 49,which has the same refractive index as outer cladding 51. The refractiveindex n₁ of the central core is less than the refractive index of thering core, whereby Δ₁<Δ₂. The term Δ is used to indicate the relativerefractive index differences between the fiber cores and the outer fibercladding. Thus, Δ₁ equals (n₁ ²-n_(c) ²)/2n₁ ² and Δ₂ equals (n₂ ²-n_(c)²)/2n₂ ², where n₁, n₂ and n_(c) are the refractive indices of thecentral core, the ring and the outer cladding, respectively. Moreover,the radal thickness w of the ring core is less than the radius r_(c) ofthe central core. The inner radius r_(R) of the ring core also affectsfilter characteristics.

Various other combinations of index deltas and radii could be employedto produce distributed resonant ring filters. In the embodiment of FIG.8, Δ₁>Δ₂, and w>r_(c). Line 59 shows that the index delta of the innercladding can be the same as that of the outer cladding (line 58);however, it can be less than or greater than the index delta of theouter cladding as indicated by lines 56 and 60, respectively.

The central core of a resonant ring fiber can support modes having mostof their power in the core. The characteristics of the ring core aresuch that it supports additional modes (LP0n) having most of their powerin the ring. Within a certain design parameter regime, the two modeschange places at the resonant wavelength λ₀; a mode (LP0m) changes froma core mode to a ring mode and a mode (LP0n) changes from a ring mode toa core mode at the same time. At this resonance, each of the two modeshas substantial power in the core and the ring. This resonance nature ofthe structure creates a strong wavelength dependent mode field patternfor LP0n and LP0m modes near the resonance wavelength, whereforeresonant ring fibers can be used as spectral filters.

Consider a structure having a refractive index profile as shown in FIG.7. With appropriate design, the fundamental mode of the structure can bethe only ring mode, the conventional core mode being LP02 mode. As thewavelength is increased, the propagation constant of the ring mode (LP01at this wavelength) decreases much faster than the core mode (LP02 modeat this wavelength). At λ₀, there is a transition between the two modes,and for λ>λ₀, the fundamental mode (LP01 mode) becomes the core mode,which now has a higher propagation constant.

To more fully understand the operating principles of the resonant ringfilter of the invention, consider the following numerically derivedexample. It is assumed that the fiber has a refractive index profile ofthe type illustrated in FIG. 7, wherein Δ₁ is 0.54%, Δ₂ is 2.5%, r_(c)is 3.83 μm, r_(r) is 15 μm and w is 0.53 μm.

FIG. 9 gives the behavior of the normalized propagation constants of theLP01 and LP02 modes. Solid line curve 65 illustrates the relationshipβ_(1-β) _(avg), and dotted curve 66 illustrates the relationshipβ₂-β_(avg), where β_(avg) is (β_(1-β) ₂)/2.

FIG. 10 gives the transformation of the LP01 mode distribution from ringmode to core mode around the resonance at λ₀. Curves 70, 71, 72, 73 and74 show the wavelength dependence of the normalized field for the LP01mode at wavelengths of 1150 nm, 1175 nm, 1200 nm, 1225 nm and 1250 nm,respectively.

The LP01 and LP02 modes are plotted at λ₀ (1205 nm) in FIG. 11 whichshows that each mode has a substantial amount of power over the core andring. In fact, the amplitude distribution of the field is the same bothover the core and ring, with the LP01 mode maintaining a same phasewhile the LP02 mode maintains an opposite phase over two regions.

At resonance, the modal field diameter is much larger than elsewhere,and therefore, the mode is more prone to bending induced loss. Acontinuous curvature is applied to the resonant ring fiber as describedabove to cause a high bending loss for light in a narrow band ofwavelengths around λ₀, but not at other wavelengths. Continuous spectralloss filters of tens of meters in length can be made this way. Filtershaving bandwidths from a few nanometers to over 100 nanometers can bemade by suitable fiber design.

A fiber draw blank can be formed in accordance with the above designcriteria; it can be drawn to a resonant ring fiber having a givendiameter; and the resonance wavelength at the peak of the loss curve ofa drawn fiber can be measured. If necessary, the resonant wavelength canbe fine tuned or changed by varying the fiber diameter with its relativeinternal structure unchanged, such that λ_(0new))=R*λ₀ where R is ratioof the fiber diameter change. This diameter change can be done on afiber drawing tower by drawing the same fiber preform into a fiberhaving a new outside diameter. Prior to drawing the modified resonantring fiber from the original draw blank, more cladding material could beadded to the original draw blank, or some cladding material could beetched from the original draw blank.

The diameter of a drawn fiber can be changed by using a set-up similarto that of a fiber optic coupler forming rig to stretch the fiber andthus obtain a constant diameter reduction over a length of fiber.

Resonant ring filters were made with refractive index profiles of thetypes shown in FIGS. 7 and 8. A FIG. 8 type fiber also included anEr-doped central core.

In one particular resonant ring fiber Δ₁ was 0.38% and Δ₂ was 1.4%. Whenthe fiber was drawn to 125 μm outside diameter, r_(c) was 4.85 μm, r_(R)was 16 μm and w was 1.02 μm; these radii and thickness w were smaller orlarger when fibers of smaller or larger outside diameter, respectively,were drawn from the same draw blank. The inner and outer claddingsconsisted of pure SiO₂, the central core was formed of SiO₂ doped withapproximately 6.1 wt. % GeO₂ and the ring core was formed of SiO₂ dopedwith approximately 21 wt. % GeO₂. The fiber was formed by well knownsoot deposition technques. A central core preform was formed by atechnique employed to make conventional step index telecommunicationsfibers. See U.S. Pat. No. 4,486,212, for example. More specifically,glass particles were deposited on a cylindrical mandrel to form a porouscore preform comprising a core region and a thin layer of claddingglass. The mandrel was removed, and the resultant tubular preform wasdried and consolidated. The resultant tubular glass article was heatedand stretched to close the axial hole and reduce the diameter thereof.Additional layers of silica cladding particles were deposited to thedesired thickness and the GeO₂-doped SiO₂ ring core particles were thendeposited, followed by a thin silica layer. After drying andconsolidating this preform, the remainder of the outer silica claddingparticles were applied and consolidated to form a draw blank from whichresonant ring fibers could be drawn.

Due to the diffusion of GeO₂ within the preforms during the manufactureof an optical fiber the actual refractive index profile is differentfrom that represented by FIGS. 7 and 8. As indicated by the plot of FIG.12, the central core region can have an index depression at the fiberaxis, and the outer edge of the core can have an index gradient. Also,the index plot of the ring core can be rounded or even pointed if itsthickness dimension w is very small.

The aforementioned draw blank was drawn into fibers having differentouter diameters to illustrate the effect of core dimensions on filterresonant wavelength. FIG. 13 shows the resonant wavelength position as afunction of fiber diameter; this graph illustrates the wide spectralrange of filters that can be formed from a single draw blank. The filterthat was drawn to an outside diameter of 135 μm exhibited peakattenuation at a wavelength of about 1620 nm. The same resonantwavelength could have been achieved at a standard outside diameter of125 μm if additional cladding material were applied during themanufacture of the draw blank. This additional cladding thickness wouldmake all core dimensions slightly smaller for a given outside diameter.

FIG. 14 shows an experimental arrangement for determining the effect ofsubjecting a resonant ring fiber 81 to different curvatures. Fiber 81,which was a 90 mm long piece of the 135 μm diameter fiber mentioned inconjunction with FIG. 13, was connected to single-core fibers 82 and 83by fusion splices s. The combination of fibers 81-83 was coiled oncearound different reels 84 having diameters D ranging from 6 cm to 15 cm.Therefore, the entire length of the resonant ring fiber 81 was subjectedto the curvature of reel 84 and functioned as a distributed loss filter.

A filter response for the different coil diameters is shown in FIG. 15where curves 91, 92, 93 and 94 represent the spectral loss exhibited byfiber 81 for values of D equal to 6 cm, 6.5 cm, 7 cm and 15 cm,respectively. Curve 95 represents the loss when fiber 81 is notsubjected to any curvature. Each of the curves 91-94 is biased 10 dBwith respect to the curve below it so that these curves can all becompared on one graph.

It is noted that there is a slight change in λ₀ at different values ofD. Ordinarily, a resonant ring fiber could be numerically designed, andonly small changes in D would be employed for tuning purposes, theresultant differnce in λ₀ being negligable. However, if the tuning ofthe attenuation required a sufficiently large change in D from thedesign value that λ₀ was adversely affected, then fiber designparameters would need to be suitably modified to move λ₀ to the correctvalue. Having thus modified the fiber design, the problem could possiblybe remedied by simply drawing a fiber of different outside diameter fromthe original draw blank. If this modification did not correct theproblem, one could make a new fiber draw blank having slightly differentcentral core and ring core.

It is thus seen that a resonant ring filter can be easily implemented toprovide the desired attenuation as well as the desired centerwavelength.

The central and ring cores are formed of a base glass such as silica andone or more dopants which are added to the base glass to produce thedesired refractive index. There are many known dopants includinggermania and alumina which, when combined with silica, produce a glasshaving a refractive index greater than that of silica. A refractiveindex decreasing dopant such as boron could be employed in combinationwith the aforementioned index increasing dopants to modify variouscharacteristics of the glass such as thermal coefficient of expansion.

A refractive index increasing dopant such as GeO₂ increases the thermalexpansion of that portion of the fiber where it is employed. Thisresults in a thermal dependence of the two propagaiton constants.Athermalization of the filter can be achieved by equalizing the thermaldependence of the propagation constants of the two modes involved. Oneway to achieve this is to use an appropriate co-dopant together with theindex raising dopant to balance out the thermal dependence of the twopropagation constants. For example, if GeO₂ is used to increase therefractive index of SiO₂ to form both the central core and the ringcore, B₂O₃ can be added to the appropriate core to balance out theeffect of GeO₂. If, for example, the central core contained 8 wt. % GeO₂and the ring core contained 20 wt. % GeO₂, then 4 wt. % B₂O₃ can beadded to the ring to achieve the necessary balance of thermal expansion.

For purposes of athermalizing the device, B_(s)O₃ could be added to boththe central core and the ring core. Preferably, the core containing thegreater GeO₂ content would also have the greater B_(s)O₃ content.

What is claimed is:
 1. A fiber optic filter comprising: an optical fiberhaving a central core, a ring core concentric with said central core, aninner cladding region of refractive index n_(i) between said central andring cores, and a cladding layer of refractive index n_(c) surroundingsaid ring core, the maximum refractive index n₁ of said central core andthe maximum refractive index n₂ of said ring core being greater thann_(c) and n_(i), wherein the maximum refractive index n₂ of said ringcore is greater than the maximum refractive index n₁ of said centralcore. the propagation constants of one core mode and one ring mode beingdifferent at wavelengths except for at least one wavelength λ₀, at leasta portion of said optical fiber being subjected to a continuouscurvature.
 2. The filter of claim 1 wherein the curvature to which saidoptical fiber is subjected is substantially circular.
 3. The filter ofclaim 1 wherein said optical fiber is wound upon a cylindrical member.4. The filter of claim 3 further comprising first and second single-corefibers, an end of each of said single-core fibers being fused to arespective end of said optical fiber.
 5. The filter of claim 4 whereinthe length of said optical fiber is less than the circumference of saidmember.
 6. The filter of claim 4 wherein the length of said opticalfiber is greater than tee circumference of said member.
 7. The filter ofclaim 1 wherein the curvature to which said optical fiber is subjectedis S-shaped.
 8. The filter of claim 1 wherein Δ₁ is greater Δ₂, where Δ₁equals (n₁ ²-n_(c) ²)/2n₁ ² and Δ₂ equals (n₂ ²-n_(c) ²)/2n₂ ².
 9. Thefilter of claim 8 wherein the radial thickness of said ring core isgreater than the radius of said central core.
 10. The filter of claim 1wherein Δ₂ is greater Δ₁, where Δ₁ equals (n₁ ²-n_(c) ²)/2n₁ ² and Δ₂equals (n₂ ²-n_(c) ²)/2n₂ ².
 11. The filter of claim 10 wherein theradial thickness of said central core is greater than the radius of saidring core.
 12. The filter of claim 1 wherein n_(i) is substantiallyequal to n_(c).
 13. The filter of claim 1 wherein said central core andsaid ring core each contain at least one refractive index increasingdopant the concentrations of said one or more dopants being such thatthe refractive indices of said central and ring cores are different. 14.The filter of claim 13 wherein at least one of said central and ringcores also contains B₂O₃.
 15. The filter of claim 14 wherein saidcentral and ring cores contain different concentrations of GeO₂, thecore having the greatest concentration of GeO₂ also containing B₂O₃.